AIWA NSX30 is a digital audio system created in the 2000s era.
At that time, so many audio brand flood Indonesian market.
This audio system has many features in it such as loading 3 CDs at once and can even be made continuously after the 3rd CD.
The tuner is available in AM and FM stereo bands, also there is an AUX input for amplifying signals from small devices such as DVD players, computers, and other sources of stereo.
This AIWA power amps are classified as small power but are power efficient because they use class G amplification techniques.
It mean namely a large supply voltage will be available if there is a fairly large signal source. When the signal is small, the amp will applying uses a low voltage supply.
SCHEMATIC DIAGRAM
The following is a schematic of the AIWA system audio that I drawing it in the power amplifier section only.
From the schematic above, it appears that this amplifier uses 2 voltages for supply.
This is -25VDC to supplying the preamp circuit and Low Power Supply.
Then a symmetrical 55 VDC to fill the Power Amplifer circuit.
The power amplifier transistor is not directly connected to a high supply voltage.
Its connected by a MOSFET which acts as a current switch for bigger audio signal.
When the audio signal low, the power transistor apply supply voltage of 25VDC Low Voltage through Diode D9, and -25VDC through D4.
The low-voltage bypass diode is RK34. ( DATASHEETS ).
Power transistors use the Darlington type with numbers FN1016 and FP1016 which have dissipation capabilities of up to 70 Watts. ( DATASHEETS ).
While the voltage driver to supply large voltages is MOSFET number 2SK2723, which has a current flow capability of up to 25 Amperes. ( DATASHEETS ).
NOTE
When the audio signal is large, it will became a voltage exceeding the emitter base threshold above 0.6V VDC on the resistors R22 and R25.
Furthermore, the signal will work to open the driver transistor and the MOSFET to flow current to the Darlington transistor.